An SSD will be up to five times faster in booting up (which there is no need for, as there is the SLEEP feature, which gives you instantaneous access to your Mac) and will let your Mac react faster to your input due to lower seeking times.
An HDD will offer much more storage capacity for less currency per GB though.
Maybe have a look at the Momentus XT from Seagate?
Btw, MRoogle is a good tool to search the fora.